Directional sio2 etch using low-temperature etchant deposition and plasma post-treatment

ABSTRACT

Methods for processing a substrate are described herein. Methods can include positioning a substrate comprising silicon in a processing chamber, delivering a plasma to the surface of the substrate while biasing the substrate, exposing the surface of the substrate to ammonium fluoride (NH 4 F), and annealing the substrate to a first temperature to sublimate one or more volatile byproducts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/744,909 (APPM/17909L02), filed Oct. 3, 2012, which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Technology described herein relates to directional etching of nativeoxides. Specifically, technology described herein relates topost-treatment of an oxide surface to selectively etch the oxidesurfaces.

2. Description of the Related Art

With the increase in transistor density and subsequent decrease in thecross-sectional dimensions of device nodes, which can be less than 22nm, pre-clean of native oxides is of particular importance. Pre-cleancan include pre-contact clean or pre-silicide clean which requiresremoval of oxides from the bottom of vias or trenches of narrowingcross-sectional dimensions. As critical dimension of semiconductordevices decreases, distances between neighboring features formed on asemiconductor substrate are also shortened. Thus, it is important tocontrol etching between vias and trenches during precleaning to preventdamaging nearby features.

Current precleaning techniques generally includes a conformal etch ofthe substrate to remove the native oxides, such as SiO₂, prior todeposition of silicides or other contacts. However, a standard conformaletch can lead to excessive cross-sectional enlargement of vias andtrenches thus creating possible leakage and ultimate device failure.Other precleaning techniques such as sputter etching can remove thenative oxides from upper surfaces. However, the sputtering can lead toredeposition of the oxides at the via or trench opening. The redepositedoxides prevent subsequent deposition in the vias and trenches leading topoor subsequent contact fill.

Thus, methods are needed to preferentially etch from the bottom surfacesof features to prevent damage to features during precleaning.

SUMMARY OF THE INVENTION

Embodiments described herein generally relate to selective etching ofnative silicon oxides. In one embodiment, a method can includepositioning a substrate comprising silicon in a processing chamber;delivering a plasma to the surface of the substrate while biasing thesubstrate; and exposing the surface of the substrate to ammoniumfluoride (NH₄F).

In another embodiment, a method can include positioning asilicon-containing substrate in a processing chamber, thesilicon-containing substrate can include an exposed surface with one ormore features formed in the exposed surface; and a native oxide layerformed on the exposed surface; cooling the substrate to a firsttemperature; exposing the surface of the substrate to ammonium fluoride(NH₄F) at the first temperature; biasing the substrate; exposing thesubstrate to a low energy inert plasma to selectively form one or morevolatile products on the top and bottom surfaces of the features;exposing the substrate to low pressure at a second temperature tosublimate the non-reacted NH₄F from the surface of the substrate; andheating the substrate to a third temperature, which is higher than thefirst and second temperature, to sublimate the one or more volatileproducts.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic top-view diagram of an illustrative multi-chamberprocessing system useable with embodiments described herein.

FIG. 2 is a diagram of a method for directional etching according to oneembodiment.

FIGS. 3A-3D are graphical representations of a substrate etchedaccording to one embodiment.

FIG. 4 depicts etch rate of the silicon oxide and silicon nitride as afunction of pedestal temperature according to one embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

Methods for removing native oxides are described herein. Precleaning ofsurfaces in vias and trenches can lead to etching of sidewalls andsubsequent reduction of cross sectional dimensions. This reduction ofcross-sectional dimensions can lead to device failure. The embodimentsdescribed herein allow for etching of surfaces to remove native oxidesfrom bottoms of vias and trenches while preserving the cross-sectionaldimensions of the via or trench. By treatment with an etchant at lowtemperatures, biasing the substrate and activation of the etchant byplasma treatment, the etchant on the bottom surfaces of trenches andvias will be etched preferentially over sidewall surfaces.

By employing a directional post-treatment of the native oxide surfaceafter to conformal exposure to an etchant at low temperatures, theetchant on the affected surface is activated for etching while thenon-activated surface is nominally etched due to desorption of theetchant before annealing. The invention is more fully explained withreference to the figures below.

FIG. 1 is a schematic top-view diagram of an illustrative multi-chamberprocessing system 200 that can be adapted to perform processes asdisclosed herein having the processing chamber 100 coupled thereto. Thesystem 200 can include one or more load lock chambers 202, 204 fortransferring substrates into and out of the system 200. Typically, sincethe system 200 is under vacuum, the load lock chambers 202, 204 can“pump down” the substrates introduced into the system 200. A first robot210 can transfer the substrates between the load lock chambers 202, 204,and a first set of one or more substrate processing chambers 212, 214,216, 100 (four are shown). The position of the processing chamber 100relative to the other chambers 212, 214, 216 us for illustration, andthe position of the processing chamber 100 may be optionally be switchedwith any one of the processing chambers 212, 214, 216 if desired.

The first robot 210 can also transfer substrates to/from one or moretransfer chambers 222, 224. The transfer chambers 222, 224 can be usedto maintain ultrahigh vacuum conditions while allowing substrates to betransferred within the system 200. A second robot 230 can transfer thesubstrates between the transfer chambers 222, 224 and a second set ofone or more processing chambers 232, 234, 236, and 238. Similar toprocessing chambers 212, 214, 216, 100, the processing chambers 232,234, 236, 238 can be outfitted to perform a variety of substrateprocessing operations.

FIG. 2 is a diagram of a method for directional etching according to oneembodiment. The method 250 can include positioning a substrate with anative oxide layer in the processing chamber, as in 252. The processingchamber can be a processing chamber as described with reference to FIG.1 or it can be a different processing chamber. The processing chambershould be capable of at least maintaining the substrate at a specifictemperature, biasing the substrate and forming NH₄F (e.g. creating NH₄Fremotely in a plasma). The substrate can be a silicon substrate withfeatures formed on the surface. The features can include one or more ofvias and trenches of varying cross-sectional dimensions, such as lessthan 27 nm. Native oxides may be formed on one or more of the surfacesof the substrate, such as a silicon dioxide formed on all exposedsurfaces. The native oxides may also be of varying thickness dependanton the circumstances of formation, such as exposure to the atmosphere.Other layers or features may be formed under the silicon oxide, such asa layer of silicon nitride or metal silicides.

The method 250 can further include cooling the substrate to a firsttemperature, as in 254. An NH₄F etching is a dry etch process forremoving one or more silicon oxides using an ammonia (NH₃) and nitrogentrifluoride (NF₃) gas mixture performed within a processing chamber. Thesubstrate is generally cooled to a temperature which allows depositionof NH₄F but below the reaction temperature. This temperature will bedependent on process conditions such as processing chamber pressure. Inone or more embodiments, the substrate can be cooled to a temperaturebelow 15° C., such as between 15° C. and 0° C., using a coolingapparatus formed within the support member. In one embodiment, thesubstrate is maintained at 10° C. In another embodiment, the substrateis maintained at a temperature of between 10° C. and 0° C.

The method 250 can further include forming ammonium fluoride (NH₄F) todeposit the etchant, as in 256. An NH₄F etching is a dry etch processfor removing one or more silicon oxides using an ammonia (NH₃) andnitrogen trifluoride (NF₃) gas mixture performed within a processingchamber. The substrate is generally cooled to a temperature below thereaction temperature of NH₄F.

The method 250 can further include exposing the substrate to NH₄Fintroduced to the chamber for removing silicon oxides on a surface ofthe substrate, as in 258. In one embodiment, ammonia and nitrogentrifluoride gases are then introduced into the plasma chamber to formthe etching gas mixture. The amount of each gas introduced into theplasma chamber is variable and may be adjusted to accommodate, forexample, the thickness of the oxide layer to be removed, the geometry ofthe substrate being cleaned, the volume capacity of the plasma, thevolume capacity of the chamber, as well as the capabilities of thevacuum system coupled to the chamber. The ratio of the etching gasmixture may be predetermined to remove various oxides on the substratesurface. The ratio of gas mixture in the etching gas mixture may beadjusted to preferentially remove the post-treated oxides, such asnative oxides formed on the top and bottom surfaces of the features. Inone embodiment, molar ratio of ammonia to nitrogen trifluoride in theetching gas mixture may be set to uniformly remove silicon oxides.

In one embodiment, etching rate of the etching gas mixture may beadjusted by adjusting a flow rate of nitrogen trifluoride whilemaintaining a molar ratio of ammonia and nitrogen trifluoride above apredetermined value. In one embodiment, etching rate may be increased ordecreased by increasing or decreasing the flow rate of nitrogentrifluoride while the ratio of ammonia and nitrogen trifluoride remainsabove about 3:1. In another embodiment, ratio of ammonia and nitrogentrifluoride can be about 1:1.

The ammonia and nitrogen trifluoride gases can be dissociated intoreactive species in a remote plasma chamber. The dissociated species cancombine to form a highly reactive ammonia fluoride (NH₄F) compoundand/or ammonium hydrogen fluoride (NH₄F.HF) in the gas phase. Thesemolecules react with the substrate surface to be processed. In oneembodiment, the carrier gas is first introduced into the chamber, aplasma of the carrier gas is generated, and then the reactive gases,ammonia and nitrogen trifluoride, are added to the plasma.

Not wishing to be bound by theory, it is believed that the etchant gas,NH₄F and/or NH₄F.HF, reacts with the silicon oxide surface to formammonium hexafluorosilicate (NH₄)₂SiF₆, NH₃, and H₂O products. The NH₃and H₂O are vapors at processing conditions and removed from the chamberby a vacuum pump. A thin film of (NH₄)₂SiF₆ is left behind on thesubstrate surface. This reaction mechanism can be summarized as follows:

NF₃+3NH₃→NH₄F+NH₄F.HF+N₂

6NH₄F+SiO₂+heat→(NH₄)₂SiF₆+2H₂O+4NH₃

(NH₄)₂SiF₆+heat→2NH₃+2HF+SiF₄

The reaction as shown above requires temperature to both form (NH₄)₂SiF₆and to sublimate the (NH₄)₂SiF₆ to NH₃, HF, SiF₄. By cooling thesubstrate below an activation, the NH₄F is present on the surface butnominally reacting with the silicon oxide.

The method 250 can further include treating the substrate with a lowenergy inert plasma while biasing the substrate, as in 260. The inertplasma can comprise any inert gas. Inert gases include noble gases, suchas helium or argon. The inert plasma is formed into a plasma ofsufficiently low energy so as to not sputter the substrate. Statedanother way, the inert plasma is a primarily ionized species so that thedirectionality is random. The inert plasma may be either formed remotelyin a plasma chamber and delivered to the chamber or formed inside thechamber itself.

The inert plasma is flowed toward the substrate which is simultaneouslybiased. The bias on the substrate can be of any power level which willnot promote sputtering, such as between 25 W and 250 W. The bias can bedelivered at varying frequencies, such as a bias of 350 kHz, 13.56 MHz,60 MHz or combinations thereof. As previously disclosed, the inertplasma is a low energy plasma which is largely ionized, thus having arandom directional movement. The bias applied to the substrate attractsthe ionized gas in the plasma toward the substrate, where the ionizedgas strikes surfaces which are perpendicular to the direction of ionizedgas movement, such as the bottom of a via or trench on a substrate. Theionized gas provides activation energy for the formation of (NH₄)₂SiF₆from NH₄F as previously disposed on the surface and SiO₂ as present inthe native silicon oxide layer.

The method 250 can further include exposing the substrate to a lowpressure while maintaining the first temperature, as in 262. After theNH₄F and SiO₂ are reacted, the non-reacted NH₄F, which is primarily onthe sidewalls of the vias and trenches, can be low pressure sublimatedat a low temperature. Low pressures can include any pressures which willallow for desorption of non-reacted NH₄F, such as a pressure of 20mTorr. The low pressure allows for desorption of the non-reacted NH₄F.Further, by keeping the temperature low, the non-reacted NH₄F will notreact with the remaining SiO₂ to create (NH₄)₂SiF₆ on undesired areas,such as on the sidewalls of trenches and vias. Non-reacted precursorsare then removed from the chamber so as to not affect furtherprocessing.

The method 250 can further include removing the (NH₄)₂SiF₆ by heatingthe substrate to a second temperature to sublimate volatile byproducts,as in 264. After the thin film is formed on the substrate surface, thesupport member may be elevated to an anneal position in close proximityto a heated gas distribution plate. The heat radiated from the gasdistribution plate may dissociate or sublimate the thin film of(NH₄)₂SiF₆ into volatile SiF₄, NH₃, and HF products. These volatileproducts are then removed from the chamber by the vacuum pump asdescribed above. Typically, a temperature of 75° C. or more is used toeffectively sublimate and remove the thin film from the substrate.Preferably, a temperature of 100° C. or more is used, such as betweenabout 115° C. and about 250° C.

The method 250 can further include flowing inert gas to evacuate thevolatile byproducts from the chamber, as in 266. The thermal energy todissociate the thin film of (NH₄)₂SiF₆ into its volatile components istransferred by the gas distribution plate through convection orradiation. In one aspect, the distribution plate is heated to atemperature of between 100° C. and 150° C., such as about 120° C.Further embodiments use a low energy plasma, such as a plasma asdescribed with reference to the post-treatment process, to enhance thesublimation of volatile byproducts. The plasma is delivered to thesurface of the substrate uniformly and at an energy level which will notsputter the oxides form the substrate. By using a low energy plasmawhile simultaneously heating the substrate, it is believed that theactivation energy for sublimation can be reduced. For example, a layerof (NH₄)₂SiF₆ may be of a certain thickness which requires a temperatureof 120° C. over a certain time period to sublimate. By using a lowenergy plasma, the layer of (NH₄)₂SiF₆ can be sublimated at 100° C. overthe same time period or at 120° C. over a shorter time period.

The method can further include making a determination of whether adesired thickness of the bottom surface has been reached, as in 268. Ifa desired etch rate has not been achieved, the substrate can be cooledto the first temperature and the process can begin again. Based on theselectivity and the directionality, the process can be repeated numeroustimes to achieve desired results, such as repeating the process 10times. Further, though the general steps are repeated in each cycle, theindividual steps can be any disclosed embodiment, without regard to theprevious embodiment chosen at that step. For example, if a helium plasmawas delivered to a substrate with a bias of 100 W in the first cycle,the second cycle could be an argon plasma delivered to the substratewith a 50 W bias.

If a desired etch rate has been achieved, the process can be ended, asin 270. The processing chamber is purged and evacuated. The processedsubstrate is then removed from the chamber by lowering the substratemember to the transfer position, de-chucking the substrate, andtransferring the substrate through a slit valve opening.

Without intending to be bound by theory, it is believed that at lowtemperatures, the chemical etch rate is practically zero because theNH₄F etchant deposits but does not form (NH₄)₂SiF₆. During a standardNH₄F etch process, the substrate will be maintained at a temperatureless than 40° C., such as a temperature between 25° C. and 40° C. Inthis temperature range, the reaction between the NH₄F and the oxidelayer on the substrate is believed to be reaction limited, such thathigher levels of reactant will lead to increased and uniform etching ofthe oxide layer. When the temperature is raised below the reactiontemperature, the deposition rate no longer reflects the formation of(NH₄)₂SiF₆. Therefore, the areas of silicon oxide have the reactantspresent but do not have the energy required to make the product. Theareas which have been post-treated with the inert plasma, however, form(NH₄)₂SiF₆.due to directional energy provided by the inert plasma. Assuch, at temperatures below 15° C., the silicon oxide is not etched inareas not targeted by inert plasma (e.g. side walls of vias andtrenches) and it is etched in targeted areas (e.g. upper surface of thesubstrate and bottoms of trenches).

Important to note is that the etching process is further selective foretching of silicon oxide over other layers which may be disposed on thesubstrate. NH₄F etchant will etch silicon oxide without substantialetching of layers such as silicon nitride or metal silicides. Theexperimentally determined selectivity between SiO₂ and SiN is greaterthan 7:1 and in some examples greater than 9:1. Selectivity of SiO₂ toSi is at least greater than 5:1. Thus, the above method provides forboth selectivity and directionality in etching of native oxides.

FIGS. 3A-3D are graphical representations of a substrate 300 etchedaccording to one or more embodiments. FIG. 3A depicts a substrate 300with a native oxide layer 303 according to one embodiment. The substrate300 can be a silicon-containing substrate, such as a crystalline siliconsubstrate. The substrate 300 has an upper surface 302. The upper surface302 has a native oxide layer 303 formed thereon, such as a silicon oxidelayer formed on a silicon-containing substrate. The native oxide layer303 can be a result of transfer between chambers (i.e., exposure toatmosphere). The substrate 300 can further have vias and trenches formedtherein, such as a via 308. The native oxide layer 303 can be depositedon sidewall surfaces 306 of features and bottom surfaces 304 offeatures. The substrate 300 can be positioned in a processing chamber asdescribed above.

FIG. 3B depicts a substrate during low temperature treatment with NH₄Fetchant according to one embodiment. The NH₄F etchant 307 can beconformally deposited on the native oxide surface 303. When this step isperformed at temperatures lower than 15° C., such as 10° C., the NH₄Fetchant will not react with the native oxide layer 303 to form(NH₄)₂SiF₆. It is preferred that the NH₄F etchant 307 be present on thesurface of the substrate for as short a time as possible to preventspontaneous formation of (NH₄)₂SiF₆.

FIG. 3C depicts a substrate 300 during plasma treatment according to oneembodiment. The substrate 300 is treated with a low energy inert plasma310, as described with reference to the embodiments above. The plasma310 can activate the NH4F etchant 307 on the top and bottom surfaces 304to form the etchant layer 312. The etchant layer comprises both(NH₄)₂SiF₆ from the activation with plasma, as well as unreacted NH₄Fetchant. The substrate should be maintained at a temperature below 15°C., such as 10° C., to prevent activity of the NH₄F etchant 307 on thesidewalls. The NH₄F etchant 307 deposited on sidewall surfaces 306 islargely unaffected by the plasma 310. The bias in the substrate 300provides directionality to the plasma 310 to prevent the plasma 310 fromtargeting the sidewall surfaces 306. The bias delivered to the substratecan be between 25 W and 200 W.

FIG. 3D depicts the substrate 300 after etching with the NH₄F etchant,according to one embodiment. After the (NH₄)₂SiF₆ film 312 is formed onand from the top and bottom surfaces 304, the remaining NH₄F etchant 307is desorbed from the surface by low pressure while maintaining thetemperature below 15° C., or preferably below 10° C. Further embodimentscan include altering pressure or temperature to optimize the desorptionof the NH₄F etchant 307 from the sidewall surface 306. In anotherembodiment, the temperature is increased to a temperature above the dewpoint of the NH₄F etchant 307, such as above 70° C. In this way, thedesorption rate is increased while preventing adsorption of the NH₄Fetchant 307 on the sidewall surfaces 306 of the substrate 300.

Next, the substrate is annealed to sublimate the (NH₄)₂SiF₆ film thusexposing the cleaned surfaces 314. The substrate 300 is heated to asecond temperature, such as a temperature higher than 75° C., withpreferred embodiments of greater than 100° C. The thickness andcomposition of sidewall surfaces 306 are substantially unchanged.

FIG. 4 depicts etch rate of the silicon oxide and silicon nitride as afunction of pedestal temperature according to one embodiment. Etch rateswere measured and plotted as shown in the graph with the oxide etch ratein A/sec. over temperature in degrees C. The silicon nitride showed nosubstantial etching at any temperature in comparison to the siliconoxide. Silicon oxide etching increases linearly in the presence of NH₄Fin temperatures between 15° C. and 30° C. From 30° C. to about 70° C.the silicon oxide surface decreases until it reaches the etch rate ofsilicon nitride. The decline in etch rate for silicon oxide after 30° C.is believed to be related to an increase in desorption of the NH₄F fromthe surface of the substrate prior to the formation of (NH₄)₂SiF₆. Attemperatures higher than 70° C., the etch rate on the untreated surfaceis substantially lower than either the prior temperature untreatedsurface etch rate or the treated surface etch rate. It is believed that,at this temperature or above, the adsorption rate and the desorptionrate are equal. Thus, a minimal amount of (NH₄)₂SiF₆ is formed on thesilicon oxide. Thus by maintaining the temperature of the deposited NH₄Feither below 15° C. or above 70° C., the formation of (NH₄)₂SiF₆ in thepresence of NH₄F on silicon oxide can be controlled for directionaletching.

CONCLUSION

Embodiments described herein relate to methods of directional removal ofnative oxides form a surface. Above embodiments show preferentialetching of post-treated surfaces over untreated surfaces. SiO₂ is formednatively on silicon surfaces and must be removed for proper depositionin vias and trenches. However, it is important to avoid changing thecross-sectional dimensions of the modern day vias and trenches, whichcan lead to device failure. A deposition of NH₄F at a low temperaturewill be effective for depositing the NH₄F conformally with limitedformation of (NH₄)₂SiF₆ on the silicon oxide. Subsequently, by treatinga biased substrate with a low energy inert plasma, the (NH₄)₂SiF₆ layerwill form preferentially on the bottom and top horizontal surfaces overthe sidewall surfaces. Subsequent annealing will be effective forremoving the plasma-treated SiO₂ without affecting the untreated SiO₂ onthe side walls of the vias or trenches.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method, comprising: positioning a substratecomprising silicon in a processing chamber; delivering a plasma to thesurface of the substrate while biasing the substrate; and exposing thesurface of the substrate to ammonium fluoride (NH₄F) or ammoniumhydrogen fluoride (NH₄F(HF)).
 2. The method of claim 1, furthercomprising cooling the surface of the substrate to a temperature at orbelow 15° C. while exposing the surface of the substrate to NH₄F.
 3. Themethod of claim 1, wherein the delivery of the plasma and the exposureto NH₄F is repeated a plurality of times before annealing the substrate.4. The method of claim 1, further comprising maintaining the substrateat temperatures at or below 15° C. while delivering the plasma.
 5. Themethod of claim 1, wherein the plasma comprises an inert plasma.
 6. Themethod of claim 1, further comprising annealing the substrate to atemperature above 100° C.
 7. The method of claim 1, wherein the NH₄F orthe NH₄F(HF) are formed by remote plasma.
 8. The method of claim 1,further comprising biasing the substrate at a frequency of from from 200kHz to 60 MHz.
 9. The method of claim 1, wherein the NH₄F or theNH₄F(HF) are formed using a gas mixture comprising a 3:1 ratio ofammonia (NH₃) and nitrogen trifluoride (NF₃) respectively.
 10. A method,comprising: positioning a silicon-containing substrate in a processingchamber, the silicon-containing substrate comprising: an exposed surfacewith one or more features formed in the exposed surface; and a nativeoxide layer formed on the exposed surface; cooling the substrate to afirst temperature at a first pressure; exposing the surface of thesubstrate to ammonium fluoride (NH₄F) at the first temperature; biasingthe substrate; exposing the substrate to a direct plasma to selectivelyform one or more volatile products on the top and bottom surfaces of thefeatures; exposing the substrate to a second pressure at a secondtemperature to sublimate the non-reacted NH₄F from the surface of thesubstrate; and heating the substrate to a third temperature, which ishigher than the first temperature, to sublimate the one or more volatileproducts.
 11. The method of claim 10, wherein the first temperature isless than 15° C.
 12. The method of claim 10, wherein the secondtemperature is greater than 70° C.
 13. The method of claim 10, whereinthe second temperature is less than 15° C.
 14. The method of claim 10,wherein the third temperature is a temperature of greater than 100° C.15. The method of claim 10, wherein a plasma comprising NH₄F is formedremotely.
 16. The method of claim 10, further comprising exposing thesubstrate to low pressure and low temperatures while delivering the lowenergy remote plasma.
 17. The method of claim 10, wherein the NH₄F isformed from a gas mixture comprising ammonia (NH₃) and nitrogentrifluoride (NF₃).
 18. The method of claim 17, wherein the gas mixtureis a 3:1 ratio of ammonia (NH₃) and nitrogen trifluoride (NF₃)respectively.
 19. The method of claim 10, wherein the surface of thesubstrate is further exposed to ammonium hydrogen fluoride (NH₄F(HF)).20. The method of claim 10, wherein second pressure is lower than thefirst pressure.